Sensitivity control for radio receivers



P 1942- e. MONTI GUARNIERI 1 2,297,470

SENSITIVITY CONTROL FOR RADIO RECEIVERS Filed April 27, 1940 J. amB

Patented Sept. 29, 1942 SENSITIVITY CONTROL FOR RADIO RECEIVERS GaetanoMonti Guarnieri, Milan, Italy; vested in the Alien Property CustodianApplication April 27, 1940, Serial N0. 332,047 In Italy May 12, 1939 2Claims.

This invention relates to means for the sensitivity control in radioreceiver sets and comprises sensitivity control means adapted to keepthe signal/noise ratio at a high value throughout the sensitivitycontrol range in respect of the noise which develops in a radio receiverset as an effect of the operation of electron tubes thereof, as wellknown.

It is known that in radio receiver sets the electron tubes and thecircuits in which they are operative, develop perturbations inherent totheir operation, said perturbations causing a noise which affects theset operation.

Said noise depends on two actions, that is: (1) on the heat effect inthe circuits which generates electromotive forces depending on anelectron unbalance in said circuits which in turn depends on thetemperature, said elfect being substantially constant so long as theimpedances of the circuit elements are unaltered; (2) on the shot effectarising in electron tubes and due to uneven and intermittent emission ofelectrons by the tube cathodes, this effect varying with the square rootof the plate circuit current in the concerned tube.

In radio receiver sets provided with sensitivity control when the setsensitivity is adjusted to a high value, the signal/noise ratio is alsohigh and accordingly the noise in the output has an admissible value,but when the sensitivity is decreased said ratio decreases because whilethe signal is :2

frequency circuits are unchanged whilst the shot :2

effect decreases in a ratio with the square root of the signal only.Consequently when the receiver sensitivity is decreased the signalaction in the set network decreases to a larger extent than the noiseand the set operation is affected.

This invention is directed to remove the above outlined objection; inthis invention the adjustment of the set sensitivity is effected,assuming its highest value as the starting point, by reducing first thesensitivity of the stage following the first one or ones and by reducingthe sensitivity of the first stage or stages only when the signal is sostrong as to require a reduction of the sensitivity of said first stageor stages; also, to avoid overloading of the respective tube or tubes.

The signal/noise ratio is thus held unchanged at its high value in thefirst stages of the set and the amplification of said noise is reducedin the subsequent stages; on the occurrence of very 55 strong signalsthe reduction of the input stages sensitivity which acts adversely inrespect of the signal/noise ratio, is accompanied by such a reduction inthe gain in the subsequent stages that the noise has practically noeffect on the set output.

This invention also includes a device comprising a single sensitivityadjusting means operative on all the stages to be controlled, said meansactuating jointly, the sensitivity controls belonging to the first stageor stages and to subsequent stages respectively to provide for thedesired effect.

An embodiment of this invention is illustrated diagrammatically in thesingle figure of the annexed drawing in connection with asuperheterodyne radio receiver of which only the essential parts areshown.

In said figure, I designates a radio frequency amplifier tube energisedby the antenna-earth circuit 2; 3 denotes the mixer tube energised bythe output of tube I by means of a transformer 4 and by a localoscillator circuit which is shown by a block 5, it being of any knownpreferred type. The tube 3 energises by means of the transformer 6 thetwo intermediate frequency tubes 1, 8 which are cascade-connected bymeans of a transformer 9, the tube 8 feeding the circuit iii of a seconddetector; the subsequent circuits of the set are not shown since theeymay be of any preferred type and are not concerned with the presentinvention.

The control grids of the tubes l, 3, l, 8 are selfbiased by the voltagedrop developed by the plate currents of the respective tubes acrossresistors Tl, 1'3, 11 and r8 connected across the tube cathodes andearth.

In the above outlined arrangement the set sensitivity is adjusted bycontrolling the grid bias in the several tubes, this control beingeffected by changing the operative value of the self bias resistors TI,r3, r1, 18.

To embody the present invention in a circuit of the above describedclass, the control of the resistor rl of the tube 1 is made separatewith respect to the control of the resistors r3, 1''! and 18 of theother tubes 3, l, 8; accordingly when the sensitivity is adjusted thevalues of resistors r3, r1, 18 only may be changed and the value of theresistor TI on the contrary may be left unaltered to secure the abovestated conditions.

Further the adjustment of the resistor TI on one hand and of theresistors 1-3, r1, r8 on the other one may be intercoupled so that thesensitivity of the input tube l is also reduced after the sensitivity ofthe tubes 3, I, 8 has been reduced through a suflicient extent.

For such a purpose in accordance with this invention the resistor H isearthed through a variable resistor II and the resistors r3, 11, 18 areinterconnected in parallel and subsequently to earth through a singleadjustable resistor I2; the sensitivity of the tubes 3, I, 8 may thus bechanged to the desired extent by adjusting the resistor I2 andsubsequently the sensitivity of the input tube I may be adjusted byadjusting the resistor II.

The adjusting means for the resistors II and I2 are with advantageintercoupled to provide for securing automatically the desiredconditions in respect of all the concerned tubes by means of a singlecontrol device.

In the illustrated embodiment the resistor TI is connected by means of alead I3 to a tap I4 adjustable along the earthed resistor II and theresistors r3, r1, T8 are connected .by means of the leads I5, I6 with amovable tap H which cooperates with the earthed resistor I2 and said tapI4 and I! are coupled with each other as shown diagrammatically at IS.

The resistor II includes on a portion of the path of the coupled movabletaps I4, II in register with the earthed end portion of the resistor I2a non-resistant or short circuited section as shown at H.

Assuming the coupled taps I4, I! to be located on the earthed ends ofthe respective resistors II, II and I2, said resistors are inoperativeand the several tubes I, 3, I, 8 have their maximum sensitivity, thebias of the respective grids being a minimum.

On the coupled taps I4, I'I being shifted from the earthed ends of therespective resistors II, II and I2, in the first portion of theiradjustment that is in the positions thereof along the inoperativesection II' of the resistor II, an increasing portion of the resistor I2is made operative; the sensitivity of the tubes 3, I, 8 is thus reducedbecause the resistances of the respective plate circuits and the bias ofthese tubes are increased but the total resistance of the cathodecircuit of the tube I and the sensitivity thereof remain unaltered.

Consequently the high signal/noise ratio is held unchanged in respect ofthe tube I while the sensitivity of tubes 3, I, 8 is reduced andaccordingly the gain they introduce in the output of tube I and theamplification of the noise are reduced; on the manipulation of the tapsI4, IT in connection with an input signal which is so strong as torequire that the sensitivity of the tube I be reduced also to avoidoverload thereof, the tap I4 reaches the operative section of theresistor II and inserts an increasing portion thereof in the platecircuit of the tube I while the tap I! by its cooperation with theresistor I2 further reduces the sensitivity of the tubes 3, I, 8 and thegain they apply to the output of the tube I and consequently to thenoise.

Under these circumstances the signal/noise ratio is increased also inthe first tube I but in this portion of the adjustment range the largereduction of the amplification of the following tubes 3, I, 8 andrespective stages deprives the noise of any substantial action on theset output.

Of course the arrangement above described in connection with the tube ofa first stage may be provided in a number of subsequent input end stagesshould it be necessary to secure a large gain and a large signal/noiseratio in the set at the same time.

This invention has been described in its embodiment in a network inwhich the sensitivity control is effected by changing the grid self biasof the amplifier tubes but it may be embodied in any suitable manner innetworks in which the sensitivity control is effected by difierentmeans.

What I claim as my invention and desire to secure by United StatesLetters Patent is:

1. In sensitivity control means for radio receiver sets, an inputsection including electron tube amplifier means and a subsequent sectionincluding electron tube amplifier means fed by said input sectionamplifier means, a cathode resistor for tube grid bias in said inputsection, a separate cathode resistor for tube grid bias in saidsubsequent section, means for adjusting said input section cathoderesistor to a certain extent and means for adjusting said subsequentsection cathode resistor to a larger extent with respect to said firstnamed extent, said adjusting means for said input section cathoderesistor being operative to increase the operative resistance value ofthe respective cathode resistor after said adjusting means for saidsubsequent section cathode resistor have been operated to a portion ofthe respective extent to increase the operative resistance value of therespective cathode resistor.

2. In sensitivity control means for radio receiver sets, an inputsection including electron tube amplifier means and a subsequent sectionincluding electron tube amplifier means fed by said input sectionamplifier means, a cathode resistor for tube grid bias in said inputsection, a separate cathode resistor for tube grid bias in saidsubsequent section, means for adjusting said input section cathoderesistor to a certain extent and means for adjusting said subsequentsection cathode resistor to a larger extent with respect to said firstnamed extent.

GAETANO MONTI GUARNIERI.

